One type of torque-control mounting system is described in U.S. Pat. No. 2,705,118 issued to M. G. Beck on Mar. 29, 1955. This configuration can be found in FIG. 4 of the '118 patent and is shown in this application as the "prior art" in FIG. 1. In the '118 Beck patent, a pair of resilient hydraulic mountings are located at opposing sides of the torque axis of a first body. The hydraulic mountings include an outer housing, an inner member and an elastomeric wall portion which is moveable in response to vibrations thereof, and a chamber within each hydraulic mounting. The working fluid is contained within the chambers of the hydraulic mountings and within a conduit for interconnecting the two hydraulic mountings. This system creates a resilient connection between a first body and a second body for restraining relative movement therebetween. This invention is particularly useful as a torque restraint between the two bodies. Depending on the orientation of the hydraulic mountings, the torque restraint can provide a system which has both a relatively low degree of freedom, i.e., relatively high stiffness, about the torque axis and a relatively high degree of freedom, i.e., relatively low stiffness, along the relative translational axis.
The teachings of U.S. Pat. No. 4,236,607 and the commonly assigned U.S. Pat. No. 4,811,919, further provide the opportunity to tune the fluid mass contained in the conduit for tuning the dynamic characteristics of the mounting system. This is accomplished by either changing the length, and/or cross sectional area of the conduit or the fluid density, in an effort to change the fluid mass value. Thus, the system can be tuned to provide improved isolation for a specified operating condition, as is well known to those skilled in the art. In addition, a restriction can be added in the conduit, as taught in the '118 Beck patent. This will allow an increase in the damping level for amplitude control along the translational axis. This restriction increases the damping by throttling the fluid through this restriction. Again, the addition of restrictions is well known to those skilled in the art.
When torque is applied to the "prior art" torque restraint system about the torque axis in the positive torque direction, the relatively incompressible fluid in the mounting chambers and conduit is placed in compression, and thus provides a relatively high restraint to rotation. For movements of the one body relative to the second along either the vertical or lateral translational axis, fluid moves from the first mounting fluid chamber through the conduit to the second mounting fluid chamber. However, it should be understood that the desirability of providing high static rotational or roll stiffness and also low translational stiffnesses in the "prior art" systems are generally competing criteria. In other words, it is difficult to obtain a system which is both statically stiff in roll and soft in translation with the system taught in the '118 Beck patent. The reason for this is clear: the need for a high static roll stiffness for restraint of roll motions necessitates a very stiff elastomeric member. The stiff elastomeric member is needed to restrict the bulge of the elastomer, i.e., a high bulge stiffness is needed. However, because of the interrelationship of the bulge stiffness and translational stiffness, this results in a higher translational stiffness than is desired. For example, in order to obtain a high bulge stiffness, stiff elastomers, thin elastomer sections, and/or a shim must be used. As a result, the lateral and vertical translational stiffnesses are higher than they would be if thick sections, soft elastomers and no shim could be used. The end result is such that if one desires to limit roll motions with the "prior art" system, then the sacrifice is in limited isolation of vibrations in the translational directions. In other words, a statically stiff roll requirement results in a higher dynamic translational stiffness than an ideal system might have.
In addition, a system which is stiff in roll, both statically and dynamically, as is the "prior art" torque restraint, will result in a body or cab reaction to every minor roll load. For example, consider a dynamically stiff torque restraint connected between a chassis and a body or cab. If the chassis receives a sudden wheel impact or impulse, such as from hitting a large bump on only one wheel, the body or cab will react with a violent roll response, because a dynamically stiff roll restraint is attached. In other words, the rotational acceleration of the body or cab will be immediately felt by the driver. In essence, the dynamically stiff roll restraint provides such a high rotational natural frequency, that it does not isolate the body or cab from low frequency roll impact forces. Therefore, it would be desirable to have a system that is statically stiff and dynamically soft rotationally, such that torque restraint can be accomplished without sacrificing torsional or roll isolation.
Torsional motions resulting from large transient applied torques can be expected to occur in both the positive and negative torque directions in real systems. For example, the roll exerted on a truck cab or railway car when cornering is assumed to be an equally clockwise and counterclockwise occurrence. For the well known "prior art" torque restraint systems, torsional deflections in one direction will act to compress the column of fluid, while in the other, they will tend to pull the fluid apart. In the latter case, when the torque is sufficiently high, and rotations about the torque axis occur, the fluid, which has a finite vapor pressure, may pull apart or "cavitate". In other words, the negative torque-carrying capability is limited in the "prior art" torque restraint. Under this condition of cavitation, the fluid torque restraint system loses stiffness, and can not support additional torques about the torque axis in the negative torque direction. This is an undesired condition because large rotational deflections will occur after the cavitation torque limit is reached. In summary, "prior art" torque restraint systems were designed to accommodate uni-directional torque. When torques are applied in the negative torque direction, only limited torque can be reacted. In addition, the "prior art" systems are deficient in terms of damping motions in the negative torque direction, once cavitation occurs.